1. Field of the Invention
The present invention relates to an x-ray computed tomography device and method, and more particularly to a high-speed, high helical pitch cone-beam x-ray computed tomography device and method.
2. Discussion of the Background
As shown in FIG. 1, an x-ray computed tomography (CT) device includes an x-ray tube 10 for irradiating a beam of x-rays, typically of cone shape, through a collimator 11, and a detector array 12 for detecting x-rays passed through an object, such as patient 13 (who typically lies on a patient bed 14). Detector 12 is positioned at a side opposite to the x-ray tube 10 with respect to the object and a tomographic image can be obtained by rotating the x-ray tube 10 integrally with the detector 12 around the object (shown by path 15) in order to scan the object and reconstruct the projected data. Typically, the tube 10 and detector 12 are circularly rotated using a gantry (not shown) while translating patient 13 to produce a helical path. A center of rotation 16 normally intersects with the approximate cross-sectional center of the object being scanned.
In helical scans the pitch is defined as the axial translation velocity of the patient table, v, multiplied by the rotation period, T. If w is the nominal slice width (the axial aperture of a single element of the x-ray detector projected at rotation center of the scanner) then the helical pitch ratio is given as EQU r.sub.H =vT/w,
the ratio of the helical translation per gantry revolution to the projected axial aperture of a detector element.
In some applications, such as patient screening and CT-angiography (CTA), throughput is more important than obtaining the best possible diagnostic image quality. In the case of CTA, shorter scan times improves imaging because of a reduction in patient motion artifacts. However, conventional CT scanners, which typically have one row of detectors, require many rotations to cover the patient volume of interest and generally keep r.sub.H .ltoreq.2. It is difficult to shorten the scan time while maintaining acceptable image quality.
Also, reconstruction is typically carried out using a back projection method with fixed integration limits. In other words, the reconstruction typically interpolates the closest beams to a reconstruction pixel.